1. Field of the Invention
The present invention relates to a method for forming a thin film and a forming apparatus therefor. More specifically, it relates to a method for successively forming a plurality of thin films by an MOCVD method and a forming apparatus therefor.
2. Description of the Related Art
The metal organic chemical vapor deposition (MOCVD) method is a well known technique to form a thin film such as a dielectric thin film, for example.
A MOCVD method thin film forming apparatus, shown in FIG. 1, is used for forming a (Ba, Sr)TiO3(BaxSr1xe2x88x92xTiO3, 0xe2x89xa6xxe2x89xa61) thin film, using, e.g., as raw materials, the following three materials; barium dipivaloyl methanate tetraethylenepentamine adduct ({Ba(C11H19O2)2(C8H23N5)2} {HN(CH2CH2NHCH2CH2NH2)2}), strontium dipivaloyl methanate tetraethylenepentamine adduct ({Sr(C11H19O2)2(C8H23N5)2} {HN(CH2CH2NHCH2CH2NH2)2}), and titanium isopropoxide (Ti(i-OC3H7)4).
The thin film forming apparatus is equipped with raw material containers 51a, 51b and 51c for containing liquid or solid raw materials, a mixer 52 for mixing raw material gases vaporized from each of the raw materials, a film deposition chamber 53 for depositing a film by the MOCVD method using the supplied raw material gas mixture mixed in the mixer 52, and a vacuum pump 54 for maintaining the inside of the above-described film deposition chamber 53 at a specific pressure (vacuum).
When the thin film forming apparatus is used for forming a (Ba, Sr)TiO3 thin film, the following process will be performed.
(1) First, a carrier gas (Ar gas in this case) is supplied to the raw material containers 51a, 51b and 51c in which the temperatures and the pressures (vacuums) are regulated at specific levels, at certain flow rates via mass flow controllers 55a, 55b and 55c, for vaporizing each of the raw materials and for supplying the thus vaporized raw material gases to the mixer 52.
(2) The raw material gas mixture mixed in the mixer 52 and containing the Ar gas is then introduced into the film deposition chamber 53 heated at a specific film deposition temperature employing the vacuum pump 54 such that the deposition chamber 53 reaches to a specific degree of vacuum.
(3) The film deposition chamber 53 is constructed so that O2 gas is introduced into the chamber as an oxidizing gas, at a given flow rate. The raw material gas mixture is introduced into the film deposition chamber 53 together with this O2 gas and blown onto a substrate 60. Through this process, the raw material gas mixture is subjected to a thermal decomposition and combustion reaction to form a (Ba, Sr)TiO3 thin film on the substrate 60.
According to the above-described conventional thin film forming method, since the melting points as well as the vaporization temperatures of the metal dipivaloyl methanate compounds are lowered (that is, the vapor pressures are raised) by forming an adduct compound with tetraethylenepentamine, it is possible to easily handle the metal dipivaloyl methanate compounds in a liquid state, and to manufacture a thin film efficiently, in comparison with the conventional method in which the metal dipivaloyl methanate compounds must be handled as raw material powders, and, therefore, are difficult to handle.
Also in the above-described thin film forming apparatus, since the carrier gas is supplied to the raw material containers 51a, 51b and 51c for vaporizing each of the raw materials, while the insides of the raw material containers 51a, 51b and 51c are evacuated and heated to specific temperatures, it is possible to bubble the carrier gas into the raw materials in order to efficiently vaporize them, and to efficiently convey and supply them to the film deposition chamber 53.
In spite of the above-explained merits, the above-described thin film forming apparatus still has drawbacks. Specifically, dipivaloyl methanate tetraethylenepentamine adduct and strontium dipivaloyl methanate tetraethylenepentamine adduct have to be heated up to 100xc2x0 C. or more in order to vaporize them for use as raw materials for MOCVD, in spite of their lowered vaporization temperatures.
Furthermore, since tetraethylenepentamine is gradually dissociated from the adduct by heating, which gradually decreases the vaporization temperatures, the vapor pressures of the raw materials (metal dipivaloyl methanate tetraethylenepentamine adduct) decrease with the passage of time. Accordingly, in order to keep the compositions of the thin films formed in plural batches of the film deposition step at a constant level, it is necessary to increase the temperature of the vaporizers, decrease the pressure of the vaporizers or increase the flow amount of the carrier gas, corresponding to each film deposition step (each batch of the film deposition).
In addition, even when these measures are adopted, dissociation of tetraethylenepentamine proceeds, causing a problem of leaving significant amounts of raw materials unusable in the raw material containers.
Furthermore, since the compositions of the raw materials change with the passage of time, there is another problem of gradual change of the properties of the thin film even when efforts are made to maintain the thin film composition constant by regulating the vaporization conditions according to the above-described method.
The present invention aims at solving the above-described problems, and, accordingly, aims at providing a forming method for forming a thin film having excellent stability of properties, which can decrease the cost for the raw materials by using them effectively, and a thin film forming apparatus therefor.
The method for forming a thin film, comprises the steps of: forming a first thin film using a raw material which comprises an adduct of metal xcex2-diketonate and adduct-forming material by a metal organic chemical vapor deposition (MOCVD) method; associating metal xcex2-diketonate dissociated from the adduct in the raw material with an adduct-forming material to regenerate the raw material; and forming a second thin film using the raw material by the MOCVD method.
The metal xcex2-diketonate is preferably a metal dipivaloyl methanate and the adduct-forming material is preferably tetraethylenepentamine.
The associating step may include the step of contacting a vapor of the adduct-forming material with a liquid of the raw material. In the case, the contacting step is performed while the liquid of the raw material is kept at a temperature lower than the that of the first or second thin film forming step. Alternatively, or in addition, the contacting step is performed while the vapor pressure of the adduct-forming material is kept higher than the vapor pressure of the raw material during the first or second thin film forming step.
The thin film forming apparatus according to the present invention comprises: a raw material container for a raw material comprising an adduct of metal xcex2-diketonate and adduct-forming material; an adduct-forming material container communicated with the raw material container such that a vapor of the adduct-forming material is capable of being supplied to the raw material container; and a film deposition chamber for depositing a thin film by an MOCVD method with a raw material gas supplied from the raw material container.
In supplying an adduct-forming material vapor from the adduct-forming material container to the raw material container, the apparatus may be constructed so that a carrier gas can be supplied to the adduct-forming material container in order to supply the adduct-forming material vapor to the raw material container together with the carrier gas, and either of the following alternatives can be chosen:
(a) supplying the carrier gas via the adduct-forming material container to the raw material container; and
(b) supplying the carrier gas to the raw material container without passing through the adduct-forming material container.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.